On my old computer with Vista, the CS was unstable when in the NPC editing window. It would crash at random times. My approach was direct frontal assault with human wave tactics. Try it over and over four times in a row if I needed to in order to succeed in saving my changes. A fresh load of the CS usually worked great. (i.e load up your mod and change the face first before changing anything else. When using the crash-prone window, save often. Perhaps after each little change or cluster of changes.
On my new computer I have Windows 7 and it is even more unstable when in the NPC editing window. It still crashes at random times, but now it crashes at some specific times too. If you see a white screen or grayed-out box RUN AWAY! Exit from that box immediately and save your mod to preserve your other work. You will probably not be able to do any major editing of NPCs during that session. You can still work on other parts of your mod, but to successfully edit NPCs you may have to exit and reload the CS. But there is no real reason to exit. You can just try to edit your NPC. If you succeed then great! (Save your work immediately before the CS can crash.) If it won't let you even try, then "Oh well." If you fail, then it will crash and you will have exited the CS. You can then reload, which is what you were contemplating doing anyway.
Vagrant0's advice sounds interesting and like it could eliminate half of my crashes. I will have to try it:
If you're using Windows 7, it's a known bug. The way to get around it is to bring the character up in the full body preview before going to the face tab.